How do I mark a seam at an edge shared by 3 faces as a seam for only two of those faces?
The problem is that the roof has no thickness - so anything you do with one side of it (add creases, set material, etc) affects the other. You need to give the roof a thickness so that it has a proper 'inside' and 'outside' - this is referred to as being 'manifold'. There are various ways of achieving this but the simplest is to go into Edit mode (Tab), select all the faces making up the roof, and hit E to Extrude, and then move the newly generated faces up to form the solid roof (hit Z to constrain movement to the Z-axis then move the mouse up).